Arabans, also known as arabinans, are polysaccharides commonly found in pectic substances. Arabans, as present in cell-wall pectic substances, have been shown to consist of a main chain of 1,5-.alpha.-linked L-arabinofuranosyl residues to which other L-arabinofuranosyl residues are linked (1,3)-.alpha. and/or (1,2)-.alpha. in either a comb-like or a ramified arrangement. Arabans are found together with other pectic substances and, when extracted by normal techniques, arabans are obtained as crude product associated with other pectic substances. Arabans may be further purified to remove substantially all associated substances by chromatographic techniques, for example as described by Tagawa et al., Carbohyd. Res., 11 (1969) 293-301. Typical crude beet araban residues contain about 70-85 wt. % arabinose, 5-10 wt. % uronic acids, 8-15 wt. % D-galactose, a few percent rhamnose and minor amounts of other monosaccharides. Purified araban normally contains no more than 2-3 wt. % of non-arabinofuranosyl residues.
It is already known to extract araban from sugar beet. For example, DD-A-143261 discloses that araban may be extracted from sugar beet material by heating the material in water in the presence of Ca(OH).sub.2 in an amount of 1-10% by weight based on the weight of beet material. The reaction mixture is maintained at 80.degree.-100.degree. C. for 10-120 minutes. L(+)-arabinose is then prepared by adjusting the pH of the mixture to 3 to 6. Sulphuric acid is added to the filtrate to a concentration of 0.2-3% and the araban is hydrolysed by heating the resultant solution at 80.degree.-100.degree. C. for 10-120 minutes. The reaction mixture is then neutralised and filtered.
Similarly, U.S. Pat. No. 4,816,078 and equivalent EP-A-276202 teach that crystalline L-arabinose may be produced from an araban-containing plant material by an extraction process using Ca(OH).sub.2. The plant material is solubilised by heating it at 105.degree.-160.degree. C. in a closed vessel in the presence of an aqueous medium containing 0.5-2 wt. % Ca(OH).sub.2, the CA(OH).sub.2 being used in an amount of from 6-17 wt. % per kg. of dry matter. The cooled reaction medium is neutralised and filtered. The filtrate is condensed by evaporation and an araban-containing fraction is obtained using an ion exchange resin. The araban is hydrolysed with 0.5-2 wt. % H.sub.2 SO.sub.4 at 92.degree.-97.degree. C. for 50-80 minutes and the reaction mixture is neutralised with CaCO.sub.3, filtered and concentrated to 40-60 wt. % dry matter by evaporation. A fraction containing L-arabinose is separated using an ion exchange resin. The arabinose-containing fraction is concentrated to 60-80% dry matter and cooled to crystallise out the arabinose.
The extraction of araban from beet pulp using CA(OH).sub.2 is also described by Tagawa et al., Methods in Enzymology, Vol. 160, Part A (1988), 542-545. Tagawa purifies the crude araban by chromatography.
Debranched araban is also a known class of material. Hitherto, it has been known principally as an undesirable constituent of fruit juices. The yield of fruit juice from apples and pears can be dramatically improved by the use of enzymes to degrade pulp polysaccharides and by more exhaustive extraction of the pulp with diffusion equipment. These processes significantly increase the amount of partially degraded polysaccharide which is solubilised. Changes in temperature or pH after extraction can lead to the precipitation or crystallisation of the partially degraded polysaccharide. Such precipitation or crystallisation gives rise to "hazy" fruit juice, which is unwelcome in the preparation of clear juices. A haze material identified in apple and pear juices is "essentially linear" 1,5-.alpha.-L-arabinan (Churms et al. (1983) Carbohyd. Res. 113, 339-344).
Churms isolated the linear L-arabinan by a process in which crude product was obtained by centrifugation and then purified by twice dispersing it in distilled water and centrifuging. The crude product was found to contain only traces of hexoses and uronic acid in addition to arabinose, whilst the purified product contained L-arabinose only. (Apple juice arabans consist almost entirely of arabinose).
It is now generally accepted that the problem of linear araban haze should be dealt with by treating the fruit pulp with pectinase enzyme preparations containing high levels of both .alpha.-L-arabinofuranosidase and endo-1,5,-.alpha.-L-arabinanase. The arabinofuranosidase cleaves the 1,3-.alpha.- and 1,2-.alpha.- linked L-arabinosyl branch units, allowing ready access of the 1,5-.alpha.-L-araban main-chain to depolymerisation by the endo-arabinanase. The implementation of this technology, however, requires the measurement of endo-arabinanase in pectinase preparations. The substrate generally employed is linear 1,5-.alpha.-L-araban (Voragen et al., (1987) Food Hydrocolloids 1,423-437) which is removed by filtration of "hazy" fruit juice concentrates.
Voragen describes a study in which the activity is investigated of a number of arabinanases when incubated with different substrates. The substrates include beet araban (highly branched), apple juice ultrafiltration retentate (UFR) araban (intermediately branched) and haze araban (linear). Voragen does not indicate that he prepared highly debranched or linear araban from beet araban and the isolation of debranched beet araban is not described. It can be inferred from Voragen that 100% debranched apple araban was not isolated, although product with a lower level of debranching was apparently isolated. Voragen does describe the isolation of arabin furanosidase enzymes.
The preparation of linear araban (1,5-.alpha.-L-araban) from purified beet araban using .alpha.-L-arabinofuranosidase is described by Tagawa et al. Carbohyd. Res., 11 (1969) 293-301. The crude beet araban used by Tagawa was purified from 73.7% L-arabinose content to 97.8% L-arabinose content, whilst the debranched material contained 98.3% L-arabinose. Tagawa does not specify the degree of debranching but it may be inferred that it was probably close to 100%. In this paper, Tagawa et al. also describe a method for the isolation of .alpha.-L-arabinofuranosidase from a culture filtrate of Aspergillus niger.
All references referred to above are incorporated herein by reference.